Systems for producing silane

ABSTRACT

Methods and systems for producing silane that use electrolysis to regenerate reactive components therein are disclosed. The methods and systems may be substantially closed-loop with respect to halogen, an alkali or alkaline earth metal and/or hydrogen.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/978,189, now U.S. Pat. No. 8,388,914 filed Dec. 23, 2010, which isincorporated herein by reference in its entirety.

BACKGROUND

The field of the present disclosure relates to methods for producingsilane and, particularly, methods which include use of electrolysis toregenerate reactive components. Some particular embodiments are directedto methods in which the production of silane is substantially“closed-loop” with respect to halogen and/or to an alkali or alkalineearth metal.

Silane is a versatile compound that has many industrial uses. In thesemiconductor industry, silane may be utilized for deposition of anepitaxial silicon layer on semiconductor wafers and for production ofpolycrystalline silicon. Polycrystalline silicon is a vital raw materialused to produce many commercial products including, for example,integrated circuits and photovoltaic (i.e., solar) cells that may beproduced by thermal decomposition of silane onto silicon particles in afluidized bed reactor.

Silane may be produced by reacting silicon tetrafluoride with an alkalior alkaline earth metal aluminum hydride such as sodium aluminumtetrahydride as disclosed in U.S. Pat. No. 4,632,816 which isincorporated herein by reference for all relevant and consistentpurposes. This process is characterized by high energy efficiency;however, starting material costs can negatively influence the economicsof such a system.

Silane may alternatively be produced by the so-called “Union CarbideProcess” in which metallurgical-grade silicon is reacted with hydrogenand silicon tetrachloride to produce trichlorosilane as described byMüller et al. in “Development and Economic Evaluation of a ReactiveDistillation Process for Silane Production,” Distillation andAdsorption: Integrated Processes, 2002, which is incorporated herein forall relevant and consistent purposes. The trichlorosilane issubsequently taken through a series of disproportionation anddistillation steps to produce a silane end-product. This processrequires a number of large recycle streams which increases the initialequipment costs as well as operating costs.

A continuing need therefore exists for economical methods for producingsilane and for methods that are closed-loop with respect to certainmaterials used within the production process. A need also exists forsystems for performing such methods including substantially closed-loopsystems.

SUMMARY

In one aspect of the present disclosure, a system for producing silanein a substantially closed-loop process includes a vessel forelectrolyzing an alkali or alkaline earth metal halide salt to producemetallic alkali or alkaline earth metal and halogen gas. The systemincludes a halogenation reactor for producing at least one of (1)silicon tetrahalide and (2) trihalosilane by reacting silicon with atleast one of (1) halogen gas discharged from the vessel and (2) hydrogenhalide produced by contacting halogen gas discharged from the vesselwith hydrogen. The system includes a hydride reactor for reactingmetallic alkali or alkaline earth metal discharged from the vessel andhydrogen to produce an alkali or alkaline earth metal hydride. Thesystem includes a silane reactor for reacting at least one of (1)silicon tetrahalide and (2) trihalosilane with the alkali or alkalineearth metal hydride to produce silane and an alkali or alkaline earthmetal halide salt.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a system for producing silane that involveselectrolysis of a halide salt according to an embodiment of the presentdisclosure;

FIG. 2 is a cross-section of a Down's cell suitable for electrolyzing ahalide salt;

FIG. 3 is a schematic of a system for producing a halogenated siliconfeed gas containing silicon tetrahalide and trihalosilane;

FIG. 4 is a schematic of a substantially closed-loop system forproducing silane according to an embodiment of the present disclosure;and

FIG. 5 is a schematic of a substantially closed-loop system forproducing polycrystalline silicon according to an embodiment of thepresent disclosure.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

Methods of embodiments of the present disclosure use electrolysis toregenerate reactive components in a process for manufacturing silane.Electrolysis allows the silane production process to optionally be asubstantially closed-loop system with respect to certain compounds usedin the system such as, for example, halogens (e.g., chlorine) and/or analkali or alkaline earth-metals (e.g., sodium). As used herein, thephrases “substantially closed-loop process” or “substantiallyclosed-loop system” refers to a process or system in which the compoundwith respect to which the system or process is closed-loop is notwithdrawn into the system or process other than as an impurity and isnot fed into the system or process for purposes other than to make-up anamount of the compound that was lost in the system as an impurity (e.g.,with the amount of compound being made-up being less than about 5% ofthe total circulating within the system as described more fully below).

In one or more embodiments of the present disclosure, silane is producedby electrolyzing an alkali or alkaline earth metal halide salt toproduce metallic alkali or alkaline earth metal and halogen gas. Themetallic alkali or alkaline metal is reacted with hydrogen to produce ahydride and the halogen gas is reacted with silicon (and additionallyhydrogen in some embodiments) to produce a halogenated silicon feed gascontaining silicon tetrahalide and, in some embodiments, trihalosilane.The feed gases are reacted to produce silane and a halide salt. Inembodiments wherein the process is substantially closed loop withrespect to at least one of the alkali or alkaline earth metals and thehalogen gas, the halide salt by-product is recycled by electrolyzing thehalide salt to produce metallic alkali or alkaline earth metal andhalogen gas.

Use of Electrolysis to Produce Silane

Referring now to FIG. 1, a halide salt 3 is introduced into a vessel 4in which the halide salt is electrolyzed to produce a halogen gas (e.g.,Cl₂) and metal (e.g., metallic alkali or alkaline earth metal). As usedherein, “halide salts” contain an alkali or alkaline earth metal and ahalogen. Halide salts may have the general formula, MX_(y), wherein M isan alkali or alkaline earth metal, X is a halogen and y is 1 when M isan alkali and y is 2 when M is an alkaline earth metal. The alkali oralkaline earth-metal of the halide salt (and which is recycled within aclosed loop system in certain embodiments as described below) may beselected from the group consisting of lithium, sodium, potassium,magnesium, barium, calcium and mixtures thereof. The halogen may beselected from fluorine, chlorine, bromine, iodine and mixtures thereof.In view of the wide availability of sodium chloride and in view thatsodium chloride may be more readily separated into its constituent parts(e.g., chloride gas and sodium metal) relative to other halide salts,sodium is a preferred alkali or alkaline earth metal and chloride is apreferred halogen. In this regard, it should be understood that anyalkali or alkaline earth metal may be used and any halogen may be used,particularly in embodiments wherein the process and system for producingsilane is a closed loop with respect to the alkali or alkaline earthmetal as described below.

One suitable vessel 4 in which the halide salt is electrolyzed is aDowns cell. An exemplary Downs cell is shown in FIG. 2 and is generallyreferenced by numeral “20”. The Downs cell 20 includes one or morehalide salts 15 therein and contains an anode 14 and cathode 16. Theanode 14 may be composed of, for example, carbon (e.g., graphite) andthe cathode 16 may composed of, for example, steel or iron. At the anode14, chlorine ions are oxidized to form a halogen gas (e.g., Cl₂). At thecathode 16, the alkali or alkaline earth metal ions are reduced to formmetallic alkali or alkaline earth metal. In this regard, it should beunderstood that as used herein, the term “metallic” refers to an alkalior alkaline earth metal with an oxidation number of 0. The halogen gasand metallic alkali or alkaline earth metal that form are divided by aseparator 19. The separator 19 may be a screen or gauze that is made ofsteel or iron. In this regard it should be understood that electrolysiscells other than Downs cells may be used such as, for example, theelectrolysis cell described in U.S. Pat. No. 5,904,821, which isincorporated herein by reference for all relevant and consistentpurposes.

The metallic alkali or alkaline earth metal that is produced is lessdense than the halide salt which causes it to rise in the cell. Thehalogen gas also rises and both the halogen gas 18 and metallic alkalior alkaline earth metal 17 are removed from the Downs cell. A secondalkali or alkaline earth metal salt may be added to the Down's cell toform a eutectic mixture and suppress the melting point of the halidesalt that is electrolyzed to reduce energy costs in melting the halidesalt and/or maintaining the halide salt in a molten state. For instance,when sodium chloride is electrolyzed in the Down's cell 20, an amount ofcalcium chloride, aluminum chloride or sodium carbonate may be added tosuppress the melting point of sodium chloride. For example, a mixturecontaining 53.2 mol % calcium chloride and 46.8 mol % sodium chloridehas a melting point of 494° C. compared to the 801° C. melting point ofsodium chloride alone and an exemplary mixture containing 23.1 mol %sodium carbonate and 76.9 mol % sodium chloride has a melting point of634° C. Preferably the alkali or alkaline earth metal of the second saltis the same as the alkali or alkaline earth metal of the halide salt oris a weaker oxidant than the alkali or alkaline earth metal of thehalide salt so as to not affect the reduction of the alkali or alkalineearth metal of the halide salt.

Referring again to FIG. 1, the halogen gas 18 is introduced into ahalogenation reactor 8 where it is contacted with silicon 6 to produce ahalogenated feed gas 21 containing silicon tetrahalide (e.g., SiCl₄).This reaction is illustrated below:Si+2X₂→SiX₄  (1)

The source of silicon 6 may be metallurgical grade silicon; however, itshould be understood that other sources of silicon may be used such as,for example, sand (i.e., SiO₂), quartz, flint, diatomite, mineralsilicates, fluorosilicates and mixtures thereof. In this regard itshould be understood that, as used herein, “contact” of two or morereactive compounds generally results in a reaction of the components andthe terms “contacting” and “reacting” are synonymous as are derivationsof these terms and these terms and their derivations should not beconsidered in a limiting sense.

As an alternative to direct reaction with silicon and as shown in FIG.3, the halogen gas 18 may be reacted with hydrogen 28 to form a hydrogenhalide 26 (HX) in a hydrogen halide burner 25 (synonymously hydrogenhalide “oven” or “furnace”). The hydrogen halide 26 may be reacted withsilicon 6 in the halogenation reactor 8 to form a halogenated siliconfeed gas 21′ containing trihalosilane and silicon tetrahalide accordingto the reactions shown below:Si+3HX→SiHX₃+H₂  (2)Si+4HX→SiX₄+2H₂  (3)The molar ratio of silicon tetrahalide to trihalosilane in thehalogenated silicon feed gas 21′ may be variable and, in variousembodiments, may be from about 1:7 to about 1:2 or from about 1:6 toabout 1:3. In this regard, it should be understood that the reaction ofsilicon 6 with hydrogen halide 26 may also produce an amount ofdihalosilane and/or monohalosilane without limitation.

In certain embodiments, reaction of halogen gas 18 with hydrogen 28 toform a hydrogen halide followed by reaction with silicon to form amixture comprising trihalosilane and silicon tetrahalide (FIG. 3) ispreferred compared to direct halogenation of silicon (FIG. 1) as lesshydride is used to produce silane from trihalosilane than silicontetrahalide as shown in Reactions 5-6ii below. Further, the directhalogenation reaction may require higher temperatures relative toreaction of hydrogen halide with silicon and may be more difficult tocontrol.

The source of hydrogen 28 may be selected from the sources describedbelow in regard to hydrogen feed 31. Optionally, the source of hydrogen28 may be hydrogen recycled with the halogenated feed gas 21′ orhydrogen that is separated from the halogenated silicon feed gas 21′.Hydrogen may be separated from the halogenated silicon feed gas 21′ byuse of a vapor-liquid separator (not shown). Examples of suchvapor-liquid separators include vessels in which the pressure and/ortemperature of the incoming gas is reduced causing the lowerboiling-point gases (e.g., silicon tetrahalide and/or trihalosilane) tocondense and separate from higher boiling point gases (e.g., hydrogen).Suitable vessels include vessels which are commonly referred to in theart as “knock-out drums.” Optionally, the vessel may be cooled topromote separation of gases. Alternatively, the hydrogen may beseparated by one or more distillation columns.

As an alternative to reaction of hydrogen and halogen in a hydrogenhalide burner followed by reaction of hydrogen halide and silicon in ahalogenation reactor as shown in FIG. 3, hydrogen gas, halogen gas andsilicon may be reacted in one vessel to produce a mixture comprisingtrihalosilane and silicon tetrahalide. In this regard, it should beunderstood that while preparation of hydrogen halide has generally beendescribed with reference to anhydrous hydrogen halide gas, in someembodiments, an aqueous hydrogen halide and, in particular, aqueous HFmay be produced which may be reacted with silicon to produce a mixturecomprising trihalosilane and silicon tetrahalide by methods known tothose of skill in the art. Further in this regard, while the reactionproduct of hydrogen halide and silicon has been described as a mixturecomprising trihalosilane and silicon tetrahalide, it should beunderstood that the reaction parameters may be controlled to producesilicon tetrahalide and only minor amounts of trihalosilane (e.g., lessthan about 5 vol % or less than about 1 vol %) or to producetrihalosilane with minor amounts of silicon tetrahalide (e.g., less thanabout 5 vol % or less than about 1 vol %).

The halogenation reactor 8 may operated as a fluidized bed in whichsilicon is suspended in the incoming gases (e.g., halogen 18 (FIG. 1) orhydrogen halide 26 (FIG. 3)). The halogenation reactor 8 may be operatedat room temperature (e.g., about 20° C.), particularly when fluorine ischosen as the halogen. More generally, the reactor may be operated at atemperature of at least about 20° C., at least about 75° C., at leastabout 150° C., at least about 250° C., at least about 500° C., at leastabout 750° C., at least about 1000° C. or at least about 1150° C. (e.g.,from about 20° C. to about 1200° C., from about 250° C. to about 1200°C. or from about 500° C. to about 1200° C.). The reactor 8 may beoperated at a pressure of at least about 1 bar, at least about 3 bar oreven at least about 6 bar (e.g., from about 1 bar to about 8 bar or fromabout 3 bar to about 8 bar).

In this regard, it should be understood that the halogenated siliconfeed stream 21 shown in FIG. 1 and halogenated silicon feed stream 21′shown in FIG. 3 may contain halosilanes other than silicon tetrahalideor trihalosilane such as amounts of monohalosilane and/or dihalosilane.Further, the halogenated silicon feed stream 21 or halogenated siliconfeed stream 21′ may be introduced into a disproportionation system (notshown) to produce amounts of trihalosilane, dihalosilane and/ormonohalosilane. It should be understood that, as used herein,“halogentated silicon feed gas” includes any gas that contains anyamount of one or more halosilanes (i.e., silicon tetrahalide,trihalosilane, dihalosilane, or monohalosilane) and includes both gasesthat have not been introduced into a disproportionation system and thathave been introduced into a disproportionation system.

Referring again to FIG. 1, the halogenated silicon feed stream 21 (orhalogenated silicon feed stream 21′ as in FIG. 3) is introduced into asilane reactor 30 to produce silane 35. Prior to introduction into thesilane reactor 30, the halogenated silicon feed gas 21 (or feed gas 21′containing both silicon tetrahalide and trihalosilane) may be purifiedto remove impurities such as, for example, aluminum halides or ironhalides (e.g., AlCl₃ and/or FeCl₃ when the halide is chlorine) and/orsilicon polymers (e.g., Si_(n)Cl_(m) polymers when the halide ischlorine). These impurities may be removed by cooling the gas toprecipitate the impurities out of the system. The precipitatedimpurities may be removed by introducing the gas into a particulateseparator such as a bag filter or cyclonic separator. To precipitate theimpurities (e.g., metal halides and/or silicon polymers) the halogenatedsilicon feed gas 21 (or silicon tetrahalide and/or trihalosilane mixture21′) may be cooled to a temperature less than about 200° C. or, as inother embodiments, less than about 175° C., less than about 150° C. oreven less than about 125° C. (e.g., from about 100° C. to about 200° C.or from about 125° C. to about 175° C.). The gas may be cooled byexchanging heat with cooling water or cooling oil in a heat exchangeapparatus and/or chiller apparatus. After impurity removal, thehalogenated silicon feed gas 21 (or silicon tetrahalide and/ortrihalosilane mixture 21′) may contain less than 10 vol % impurities(i.e., compounds other than halosilanes) or even less than about 5 vol%, less than about 1 vol %, less than about 0.1 vol % or even less thanabout 0.01 vol % impurities (e.g., from 0.001 vol % to about 10 vol % orfrom about 0.001 vol % to about 1 vol %).

The metallic alkali or alkaline earth metal 17 that is produced as anelectrolysis product is introduced into a hydride reactor 9. An amountof hydrogen 31 is also introduced into the hydride reactor 9. Reactionbetween metallic alkali or alkaline earth metal and hydrogen produces analkali or alkaline earth metal hydride 32 as shown in the reactionbelow:(2/y)M+H₂→(2/y)MH_(y)  (4)wherein y is 1 when M is an alkali and y is 2 when M is an alkalineearth metal. For instance, when M is Na, the reaction proceeds asfollows,2Na+H₂→2NaH  (4i).When M is Ca, the reaction proceeds as follows,Ca+H₂→CaH₂  (4ii).

Reaction (4) may occur in the presence of a solvent within the hydridereactor 9. Suitable solvents include various hydrocarbon compounds suchas toluene, dimethyl ether, diglyme and ionic liquids such as NaAlCl₄.In embodiments where NaAlCl₄ is used, the hydride reactor 9 may includeelectrodes. Once the supply of alkali or alkaline earth metal hydride isexhausted, the electrodes may be energized to cause sodium (including anamount of sodium from NaAlCl₄) and H₂ to react and regenerate thehydride compound. In embodiments wherein NaAlCl₄ is used as a solvent,other ionic compounds may be added to form a eutectic mixture asdisclosed in U.S. Pat. No. 6,482,381, which is incorporated herein forall relevant and consistent purposes.

The hydride reactor 9 may be a stirred tank reactor to which an amountof solvent (not shown) and metallic alkali or alkaline earth metal 17 isadded. Hydrogen 31 may be bubbled through the reaction mixture to formthe alkali or alkaline earth metal hydride 32 in batch-mode or in asemi-continuous or continuous process. Suitable sources of hydrogen 31include hydrogen obtained commercially or hydrogen obtained from otherprocess streams. For instance, hydrogen may be separated (e.g.,vapor-liquid separator as described above) from the trihalosilane andsilicon tetrahalide mixture 21′ in embodiments where hydrogen halide isreacted with silicon. Alternatively or in addition, hydrogen releasedfrom silane during the downstream production of polycrystalline siliconmay be used. The amount of solvent, hydrogen 31 and metallic alkali oralkaline earth-metal 17 added to the reactor 9 may be chosen such thatthe weight ratio of the amount of hydride to the solvent in the reactor9 may be at least about 1:20 and, in other embodiments, at least about1:10, at least about 1:5, at least about 1:3, at least about 2:3 or evenat least about 1:1 (e.g., from about 1:20 to about 1:1 or from about1:10 to about 2:3).

In one or more embodiments, the reaction mixture in reactor 9 iswell-mixed using, for example one or more a relatively high agitationmixers, having one or more impellers. Relatively high agitation allowsthe hydrogen to be well dispersed throughout the reaction mixture so asto maximize the rate of hydrogen dissolution and also shears any solidalkali or alkaline earth metal hydride from the metallic alkali oralkaline earth metal so as to allow the liquid alkali or alkaline earthmetal to be continuously available to react with the dissolved hydrogen.In this regard and without being bound to any particular theory, masstransfer in the hydride reactor depends on liquid side resistance withthe volumetric gas-liquid mass transfer coefficient (K_(L)a_(G))expected to be between about 100 to about 100,0000 s⁻¹ and, moretypically, between about 1,000 and about 10,000 s⁻¹. It should be notedthat the particular volumetric gas-liquid mass transfer coefficient(K_(L)a_(G)) may vary depending on the particular hydride and solventchosen for use in the reactor 9. Such values may be readily bedetermined by those of skill in the art according to known methodologies(e.g., measuring hydrogen uptake as a function of time).

In several embodiments of the present disclosure, the hydride reactor 9is operated under high pressure conditions such as pressures of at leastabout 50 bar, at least about 125 bar, at least about 200 bar, at leastabout 275 bar or at least about 350 bar (e.g., from about 50 bar toabout 350 bar or from about 50 bar to about 200 bar). The hydridereactor 9 may be operated at a temperature less than the thermaldecomposition of alkali or alkaline earth metal halide such astemperatures less than about 160° C., less than about 145° C. or lessthan about 130° C. (e.g., from about 120° C. to about 160° C.).

The alkali or alkaline earth metal hydride 32 is typically a solid inorganic solvents. A slurry containing the alkali or alkaline earth metalhydride 32 suspended in the solvent may be introduced into the silanereactor 30 to produce silane 35. In this regard, it should be understoodthat in certain other embodiments of the present disclosure, the alkalior alkaline earth metal hydride 32 may be introduced into the silanereactor 30 as a solid or caked solid which contain lesser amounts ofsolvent. The alkali or alkaline earth metal may be separated from thesolvent by centrifugation or filtration or by any other suitable methodavailable to those of skill in the art. In this regard, it should beunderstood that solvents other than organic solvents (e.g., NaAlCl₄) maybe used without limitation.

As described above, silicon tetrahalide from the halogenated siliconfeed gas 21 (or a mixture comprising silicon tetrahalide andtrihalosilane 21′ as in FIG. 3) and alkali or alkaline earth metalhydride 32 are introduced into a silane reactor 30 to produce silane 35and halide salt 37 according to the reactions shown below:(4/y)MH_(y)+SiX₄→(4/y)MX_(y)+SiH₄  (5)3MH_(y) +ySiHX₃→3MX_(y) +ySiH₄  (6)wherein y is 1 when M is an alkali and y is 2 when M is an alkalineearth metal. For instance, when M is Na and X is Cl, the reactionsproceed as follows,4NaH+SiCl₄→4NaCl+SiH₄  (5i)3NaH+SiHCl₃→3NaCl+SiH₄  (6i).When M is Ba and X is Cl, the reactions proceed as follows,2BaH₂+SiCl₄→2BaCl₂+SiH₄  (5ii)3BaH₂+2SiHCl₃→3BaCl₂+2SiH₄  (6ii).

The silane reactor 30 may be a stirred tank reactor (e.g., impelleragitated). The alkali or alkaline earth metal hydride 32 added to thereactor 30 may be suspended in an amount of the solvent (e.g., toluene)in which it was produced (e.g., by reaction of an alkali or alkalineearth metal and hydrogen). The silicon tetrahalide and/or trihalosilane31 may be bubbled through the hydride slurry and, preferably, in acounter-current relationship. The weight ratio of hydride 32 added tothe reactor 30 to the amount of solvent added to the reactor may be atleast about 1:20 and, in other embodiments, at least about 1:10 or atleast about 1:5 (e.g., from about 1:20 to about 1:5 or from about 1:20to about 2:10). Silicon tetrahalide from halogenated silicon feed gas 21(FIG. 1) or silicon tetrahalide and trihalosilane from halogenatedsilicon feed gas 21′ (FIG. 3) may be added in a substantiallystoichiometric ratio relative to hydride 32 with the molar ratios beingshown in reactions (5) to (6ii) above.

An amount of catalyst such as, for example, tri-ethyl aluminum, variouslewis acids or trace alkali metals (e.g., impurity lewis acids such asmetal chlorides) may be added to the reactor 30. Such catalysts reducethe temperature at which reactions (5) and (6) achieve sufficientconversion and may reduce the amount of heat input into the system. Inembodiments wherein a catalyst is not employed, the reactor 30 may beoperated at temperatures of at least about 120° C. (e.g., from about120° C. to about 225° C. or from about 140° C. to about 200° C.);whereas, in embodiments wherein a catalyst is used, the reactor 30 maybe operated at a relatively cooler temperature of at least about 30° C.(e.g., from about 30° C. to about 125° C., from about 40° C. to about100° C. or from about 40° C. to about 80° C.). The average residencetime for materials added to the reactor 30 may be from about 5 minutesto about 60 minutes.

The silane gas 35 may be relatively pure (e.g., contain less than about5 vol % or even less than about 2 vol % compounds other than silane).After silane gas 35 is removed from the reactor 30, the silane gas 35may be subjected to further processing. For example, silane 35 may bepurified (e.g., to remove compounds such as boron halides or phosphoroushalides) by introduction into one or more distillation columns and/ormolecular sieves to remove impurities as disclosed in U.S. Pat. No.5,211,931, U.S. Pat. No. 4,554,141 or U.S. Pat. No. 5,206,004, each ofwhich is incorporated herein by reference for all relevant andconsistent purposes, or by any of the other known methods available tothose of skill in the art.

The silane gas 35 may be used to prepare polycrystalline silicon (e.g.,granular or chunk polycrystalline silicon) or may be used to prepare oneor more epitaxial layers on silicon wafers. The silane gas may be storedand/or transported before use as appreciated by those of skill in theart.

The reaction of alkali or alkaline earth-metal hydride and silicontetrahalide in the halogenated silicon feed gas 21 (or mixture 21′comprising trihalosilane and silicon tetrahalide) produces an alkali oralkaline earth metal halide salt 37 as a by-product. The halide salt 37may be dissolved and, more typically, suspended in the solvent (e.g.,toluene) in embodiments where solvents are used. The halide salt 37 maybe separated from the solvent and sold commercially or recycled for useas further described below.

Substantially Closed-Loop Process for Producing Silane

The process described above may be incorporated into a substantiallyclosed-loop process for producing silane. The process above may beclosed-loop with respect to alkali or alkaline-earth metals and/or withrespect to halogens. Referring now to FIG. 4, the halide salt 37 may beseparated from solvent by use of a separator 40. The separator 40 may bean evaporator or other suitable equipment may be used includingcrystallizers, filtration and/or gravity-based separators (e.g.,centrifuges) in addition or alternatively to an evaporator. Suitableevaporators include wiped-film evaporators. After separation, the driedhalide salt may be heated (e.g., up to 500° C.) to remove trace solvent.

The solvent 43 may be condensed and reintroduced into the hydridereactor 9 and/or into the silane reactor 30. The separated halide salt 3may be used as the feed 3 for electrolysis such that alkali or alkalineearth-metal and/or halide is substantially recycled throughout thesystem.

In this regard, it should be understood that the substantiallyclosed-loop process shown in FIG. 4 may be modified to include ahydrogen halide burner 25 to produce a gas 21′ containing silicontetrahalide and trihalosilane as in FIG. 3.

As shown in FIG. 4, the process is substantially closed loop withrespect to alkali or alkaline earth-metal and with respect to halogensin that the system does include alkali or alkaline earth-metal orhalogen (i.e., either alone or as within alkali or alkaline earth metalor halogen containing compounds) in any of the inlet streams 6, 31 andin that alkali or alkaline earth-metal and halogen are not removed inoutlet stream 35. In this respect, it should be understood that alkalineor alkaline earth metals and/or halogens may be removed from the systemas an impurity or may be included in a purge stream and may be fed intothe system or process as in a make-up stream. Any make-up of alkali oralkaline earth metal and/or halogen may be achieved by addition to thesystem of compounds which contain the respective elements and, incertain embodiments, by the respective halide salt itself. In variousembodiments, the amount of alkali or alkaline earth-metal and/or halogengas made up to the system (which may be added as an alkali or alkalineearth metal salt) is less than about 5% of the total circulating withinthe system and, in other embodiments, less than about 2% of the totalcirculating within the system (e.g., from about 0.5% to about 5%).

In some embodiments of the present disclosure, the system and processmay be substantially closed loop with respect to hydrogen. For instance,as shown in FIG. 5, silane 35 exiting the silane reactor 30 may beintroduced into a polycrystalline reactor 50, preferably afterpurification to remove trace silanes, carbon compounds, trace metals andany dopant boron, phosphorous or aluminum compounds (e.g., by cryogenicactivated carbon adsorbers). The polycrystalline reactor 50 may be afluidized bed (e.g., to produce granular polycrystalline silicon) or aSiemens reactor having electrically heated rods for thermallydecomposing silane (e.g., to produce chunk polycrystalline silicon) ormay incorporate any other reactor design suitable for producingpolycrystalline silicon. Silane thermally decomposes to producepolycrystalline silicon according to the following reaction:SiH₄→Si+2H₂

Silane may undergo further processing such as various purification stepsas described above before addition to the polycrystalline reactor 50.The reaction products of the reactor 50 include polycrystalline silicon52 and hydrogen 31. As shown in FIG. 5, hydrogen 31 is introduced intothe hydride reactor 9. Hydrogen 31 may be further processed byseparating out silicon dust and by purification (e.g., distillation)prior to introduction into the hydride reactor 9. As shown in FIG. 5,the only input into the system is silicon 6 and the only output ispolycrystalline silicon 52. The system is substantially closed loop withrespect to hydrogen in that hydrogen is only removed as an impurity oras a purge stream (not shown) and is only added as a make-up stream (notshown).

Substantially Closed-Loop System for Producing Silane

The processes of the present invention may be carried out in a systemfor producing silane, such as, for example, any one of the systemsillustrated in FIGS. 1-5. The system may be substantially closed-loopwith respect to one or more of halogens, alkali or alkaline earth metaland hydrogen.

Referring to FIG. 1, the system may include a vessel 4 (e.g., a Downscell) for electrolyzing a halide salt to produce metallic alkali oralkaline earth metal and halogen gas. The halogen gas is conveyed by aconveying apparatus to at least one of (1) a hydrogen halide burner 25to be reacted with hydrogen and produce hydrogen halide (FIG. 3) and (2)halogenation reactor 8 to react with silicon (which is conveyed by aconveying apparatus form silicon storage to the halogenation reactor 8)and produce silicon tetrahalide. In embodiments wherein halogen gas isreacted to produce hydrogen halide, the hydrogen halide may then byconveyed by a conveying apparatus to the halogenation reactor 8 toproduce a mixture comprising silicon tetrahalide and trihalosilane. Anysilicon tetrahalide and/or trihalosilane gas that is produced isconveyed by a conveying apparatus to a silane reactor 30.

The system also includes a hydride reactor 9 (e.g., stirred tankreactor). The metallic alkali or alkaline earth metal is conveyed fromthe vessel by a conveying apparatus to the hydride reactor 9. Hydrogengas is also conveyed by a conveying apparatus to the hydride reactor 9to react with the metallic alkali or alkaline earth metal to produce analkali or alkaline earth metal hydride. The system includes a silanereactor 30 (e.g., a stirred-tank reactor) to which the hydride (anysolvent, if any) is conveyed by a conveying apparatus. The hydridereacts with the silicon tetrahalide and/or trihalosilane gas to formhalide salt in the silane reactor 30, optionally in the presence of asolvent.

In various embodiments and as shown in FIG. 4, the solvent and halidesalt may be conveyed by a conveying apparatus to a separator 40 forseparating any solvent from the halide salt. The solvent may be conveyedby a conveying apparatus to the hydride reactor 9 and the halide saltmay be conveyed by a conveying apparatus to the vessel 4 (e.g., Downscell) for recycle and to complete the substantially closed-loop systemwith respect to halogen and alkali or alkaline earth metal.

In several further embodiments, the system also includes apolycrystalline reactor 50 which may be a Siemens-type reactor (i.e., areactor having electrically heated rods for thermally decomposingsilane) or a fluidized bed reactor. Silane is conveyed from the silanereactor 30 to the polycrystalline reactor 50 by a conveying apparatus toproduce hydrogen and polycrystalline silicon. Hydrogen may be conveyedby a conveying apparatus from the polycrystalline reactor 50 to thehydride reactor 9 to recycle hydrogen and complete the substantiallyclosed-loop system with respect to hydrogen.

Suitable conveying apparatus are conventional and well known in the art.Suitable conveying apparatus for the transfer of gases include, forexample, compressor or blower and suitable conveying apparatus fortransfer of solids include, for example, drag, screw, belt and pneumaticconveyors. In this regard, it should be understood that, use of thephrase “conveying apparatus” herein is not meant to imply directtransfer from one unit of the system to another but rather only that thematerial is transferred from unit to another by any number of indirecttransfer parts and/or mechanisms. For instance, material from one unitmay be conveyed to further processing units (e.g., purification orstorage units used to provide a buffer between continuous or batch-wiseprocesses) and then conveyed to the second unit. In this example, eachunit of conveyance including the intermediate processing equipmentitself may be considered to be the “conveying apparatus” and the phrase“conveying apparatus” should not be considered in a limiting sense.

Preferably, all equipment utilized in the system for producing silane isresistant to corrosion in an environment that includes exposure tocompounds used and produced within the system. Suitable materials ofconstruction are conventional and well-known in the field of thedisclosure and include, for example, carbon steel, stainless steel,MONEL alloys, INCONEL alloys, HASTELLOY alloys, nickel and non-metallicmaterials such as quartz (i.e., glass), and fluorinated polymers such asTEFLON, KEL-F, VITON, KALREZ and AFLAS.

It should be understood that the processes and systems described abovemay include more than one of any of the recited units (e.g., reactorsand/or separation units) and that multiple units may be operated inseries and/or in parallel without departing from the scope of thepresent disclosure. Further in this regard, it should be understood thatthe process and systems that are described are exemplary and theprocesses and systems may include additional units which carryadditional functions without limitation.

When introducing elements of the present disclosure or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above apparatus and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying figures shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A system for producing silane in a substantiallyclosed-loop process, the system comprising: a vessel for electrolyzingan alkali or alkaline earth metal halide salt to produce metallic alkalior alkaline earth metal and halogen gas; a halogenation reactor forproducing at least one of (1) silicon tetrahalide, and (2)trihalosilane; by reacting silicon with at least one of: (1) halogen gasdischarged from the vessel; and (2) hydrogen halide produced bycontacting halogen gas discharged from the vessel with hydrogen; ahydride reactor in combination with a reaction mixture containing asolvent and metallic alkali or alkaline earth metal, wherein metallicalkali or alkaline earth metal discharged from the vessel is reactedwith hydrogen to produce an alkali or alkaline earth metal hydride, thehydride reactor being a stirred-tank reactor in which hydrogen isbubbled through the reaction mixture to produce alkali or alkaline earthmetal hydride suspended in the solvent; and a silane reactor forreacting at least one of (1) silicon tetrahalide, and (2) trihalosilanewith the alkali or alkaline earth metal hydride to produce silane and analkali or alkaline earth metal halide salt, the silane reactor beingseparate from the hydride reactor.
 2. The system as set forth in claim 1wherein the vessel is an electrolysis cell.
 3. The system as set forthin claim 1 wherein the halogenation reactor is a fluidized bed reactorin which silicon is suspended in a halogenated silicon feed gascomprising at least one halosilane selected from the group consisting ofsilicon tetrahalide, trihalosilane, dihalosilane and monohalosilane. 4.The system as set forth in claim 1 comprising a conveying apparatus forconveying alkali or alkaline earth metal hydride and solvent from thehydride reactor to the silane reactor.
 5. The system as set forth inclaim 1 wherein the system comprises a polycrystalline silicon reactorfor decomposing silane to produce hydrogen and polycrystalline silicon.6. The system as set forth in claim 5 wherein the polycrystallinereactor comprises electrically heated rods for thermally decomposingsilane.
 7. The system as set forth in claim 5 wherein thepolycrystalline reactor is a fluidized bed reactor in which silanefluidizes polycrystalline silicon particles.
 8. The system as set forthin claim 5 wherein the system comprises a conveying apparatus forconveying hydrogen from the polycrystalline silicon reactor to thehydride reactor.
 9. The system as set forth in claim 5 wherein thesystem comprises a conveying apparatus for conveying silane from thesilane reactor to the polycrystalline silicon reactor.
 10. The system asset forth in claim 1 wherein the hydride reactor does not include aninlet for introducing silicon tetrahalide or trihalosilane.
 11. Thesystem as set forth in claim 1 comprising a conveying apparatus forconveying the alkali or alkaline earth metal hydride from the hydridereactor to the silane reactor.
 12. The system as set forth in claim 11comprising the conveying apparatus in combination with the alkali oralkaline earth metal hydride.
 13. A system for producing silane in asubstantially closed-loop process, the system comprising: a vessel forelectrolyzing an alkali or alkaline earth metal halide salt to producemetallic alkali or alkaline earth metal and halogen gas; a halogenationreactor for producing at least one of (1) silicon tetrahalide, and (2)trihalosilane; by reacting silicon with at least one of: (1) halogen gasdischarged from the vessel; and (2) hydrogen halide produced bycontacting halogen gas discharged from the vessel with hydrogen; ahydride reactor for reacting metallic alkali or alkaline earth metaldischarged from the vessel and hydrogen to produce an alkali or alkalineearth metal hydride; a silane reactor in combination with a reactionmixture containing a solvent and alkali or alkaline earth metal hydridedispersed in the solvent, wherein at least one of (1) silicontetrahalide, and (2) trihalosilane is reacted with the alkali oralkaline earth metal hydride to produce silane and an alkali or alkalineearth metal halide salt, the silane reactor being a stirred tank reactorin which a halogenated silicon feed gas comprising at least onehalosilane selected from the group consisting of silicon tetrahalide andtrihalosilane is bubbled through the reaction mixture and wherein thealkali or alkaline earth metal halide salt that is produced is dissolvedor suspended in the solvent.
 14. The system as set forth in claim 13wherein the vessel is an electrolysis cell.
 15. The system as set forthin claim 13 wherein the system comprises a conveying apparatus forconveying alkali or alkaline earth metal halide salt discharged from thesilane reactor to the vessel for electrolyzing an alkali or alkalineearth metal halide salt.